FILE - In this Tuesday, Feb. 25, 2014 file photo, children wearing masks walk home after school in Beijing, China. Air pollution kills about 7 million people worldwide every year according to a new report from the World Health Organization published Tuesday, March 25, 2014. The agency said air pollution triggers about 1 in 8 deaths and has now become the single biggest environmental health risk, ahead of other dangers like second-hand smoke. (AP Photo/Ng Han Guan, File)